1. Field of the Invention
The present invention relates to handheld type four-cycle engines which are mainly used as a power source for machines for portable operation such as trimmers. More particularly, it relates to improvement of a four-cycle engine that includes an engine main body, the engine body including a crankcase having a crank chamber, a cylinder block having a cylinder bore and a cylinder head having an intake port and an exhaust port; a crankshaft supported in the crankcase and housed inside the crank chamber; a piston fitted in the cylinder bore and connected to the crankshaft; an intake valve and an exhaust valve for opening and closing the intake port and exhaust port, the intake valve and exhaust valve being mounted to the cylinder head; a valve operation mechanism operable in association with the rotation of the crankshaft so as to open and close the intake valve and exhaust valve; and a power output or takeoff mechanism provided on one end of the crankshaft.
2. Description of the Related Art
Such a handheld type four-cycle engine is already known as disclosed in, for example, Japanese Patent Application Laid-open No. 10-288019.
Handheld type four-cycle engines are of course useful in terms of the prevention of environmental pollution as well as assuring the operators"" health since the exhaust gas is comparatively clean. However, since the structure thereof is more complicated than that of two-cycle engines, there is a drawback that it is difficult to reduce the weight thereof. Reduction in weight is an important issue particularly for improvements in the operability of handheld four-cycle engines.
However, in the handheld type four-cycle engine disclosed in the above-mentioned patent publication, since a lubricating oil passage providing communication between the crank chamber and the valve operation mechanism is formed in a side wall of the engine main body in order to lubricate the valve operation mechanism for opening and closing the intake and exhaust valves provided in the cylinder head, the thickness of the side wall of the engine main body inevitably increases so enlarging the size thereof and thus making it difficult to reduce the weight of the engine.
The present invention has been carried out in view of the above-mentioned circumstances, and it is an object of the present invention to provide a lightweight handheld type four-cycle engine having good operability by making the engine main body compact.
In accordance with a first aspect of the present invention in order to achieve the above-mentioned objective, there is proposed a handheld type four-cycle engine including an engine main body, the engine main body including a crankcase having a crank chamber, a cylinder block having a cylinder bore and a cylinder head having an intake port and an exhaust port; a crankshaft supported in the crankcase and housed inside the crank chamber; a piston fitted inside the cylinder bore and connected to the crankshaft; an intake valve and an exhaust valve for opening and closing the intake port and exhaust port, the intake valve and the exhaust valve being mounted in the cylinder head; a valve operation mechanism operable in association with the rotation of the crankshaft so as to open and close the intake valve and the exhaust valve; and a power output mechanism provided on one end of the crankshaft projecting out of the engine main body, wherein a lubrication system includes an oil tank placed outside the engine body and storing lubricating oil; a through hole providing communication between the oil tank and the crank chamber; an oil feed pipe placed outside the engine main body and providing communication between the crank chamber and a valve operation chamber, the valve operation chamber being formed in the cylinder head so as to house the valve operation mechanism; an oil return pipe also placed outside the engine main body and providing communication between the valve operation chamber and the oil tank; and transfer means for transferring the oil inside the oil tank to the oil feed pipe via the crank chamber.
The above-mentioned power output mechanism corresponds to the centrifugal clutch described in the embodiment below, and the transfer means corresponds to the one-way valve 61 in the embodiment.
In accordance with the above-mentioned first characteristic, since the oil feed pipe and the oil return pipe are placed outside the engine main body, it is possible to make the side walls of the engine main body thinner regardless of the presence of these pipes, and the engine main body can thus be made compact so achieving a great reduction in the weight of the whole engine. Moreover, the externally placed oil feed pipe and oil return pipe are less influenced by heat from the engine main body, and it is thus possible to prevent the lubricating oil from becoming overheated.
In accordance with a second aspect of the present invention, in addition to the above-mentioned first characteristic, there is proposed a handheld type four-cycle engine wherein oil mist generation means for generating an oil mist from the stored oil is provided inside the oil tank, and the transfer means for transferring the oil mist generated inside the oil tank to the oil feed pipe includes valve means for introducing the positive pressure component of pressure pulsations of the crank chamber to the oil feed pipe.
The above-mentioned valve means corresponds to the one-way valve 61 described in the embodiment below.
In accordance with the above-mentioned second characteristic, since the oil mist generated in the oil tank is supplied to the crank chamber and the valve operation chamber by utilising the pressure pulsations of the crank chamber and is further returned to the oil tank 40, the inside of the engine can be effectively lubricated in any operational position of the engine and, moreover, a special oil pump for circulating the oil mist is unnecessary and the structure can thus be simplified.
In accordance with a third aspect of the present invention, in addition to the above-mentioned first or second characteristic, there is proposed a handheld type four-cycle engine wherein the oil feed pipe and the oil return pipe are formed integrally with a belt cover provided between the outside face of the engine main body and a timing transmission of the valve operation mechanism.
In accordance with the above-mentioned third characteristic, the integral formation of the oil feed pipe and the oil return pipe with the belt cover can contribute to a reduction in the number of parts and an enhancement of the assembly performance
In accordance with a fourth aspect of the present invention, in addition to the above-mentioned first or second characteristic, there is proposed a handheld type four-cycle engine wherein the oil feed pipe and the oil return pipe include flexible tubes.
In accordance with the above-mentioned fourth characteristic, the oil feed pipe and the oil return pipe can be freely fitted to connection points, wherever the points are, by appropriately flexing these pipes, and the degrees of freedom of the layout can be increased.
In accordance with a fifth aspect of the present invention, in addition to the above-mentioned first characteristic, there is proposed a handheld type four-cycle engine wherein the valve operation mechanism includes a camshaft supported in a rotatable manner in the cylinder head so as to open and close the intake valve and the exhaust valve, and a dry system timing transmission placed outside the engine main body and operable in association with the crankshaft to the camshaft; oil mist generation means for generating an oil mist from the stored oil is provided inside the oil tank; and the transfer means for transferring the oil mist generated inside the oil tank to the oil feed pipe includes valve means for introducing the positive pressure component of pressure pulsations of the crank chamber to the oil feed pipe.
In accordance with the above-mentioned fifth characteristic, since the engine is made in the form of an OHC type, and the timing transmission system is made in the form of a dry system and placed outside the engine main body, it is unnecessary to specially provide a transmission chamber for housing the timing transmission on the side wall of the engine main body and it is therefore possible to make the engine main body thin and compact and to reduce the overall weight of the engine to a great extent. Since the valve means feeds the positive pressure component of the pressure pulsations of the crank chamber to the valve operation chamber side, the oil mist generated in the oil tank on the engine main body side is circulated to the crank chamber, the valve operation chamber and the oil tank via the oil feed pipe and the oil return pipe so lubricating the inside of the engine in any operational position of the engine. Moreover, it is unnecessary to provide a special oil pump for the circulation of oil so contributing to a simplification of the structure and, as a result, a reduction in the cost.
Furthermore, in accordance with a sixth aspect of the present invention, in addition to the above-mentioned second or fifth characteristic, there is proposed a handheld type four-cycle engine wherein a suction chamber adjoining the upper part of the valve operation chamber is provided in the cylinder head, the oil return pipe being connected to the suction chamber, and the suction chamber is communicated with to the valve operation chamber via a plurality of orifices at different height levels.
In accordance with the above-mentioned sixth characteristic, even if the oil mist liquefies and resides in the valve operation chamber, this liquefied oil can be returned to the oil tank by drawing it up into the suction chamber via one of the orifices regardless of the positional state of the engine such as an upright or upside down state, and it is thus possible to prevent oil remaining in the valve operation chamber.